1. The Field of the Invention
This invention relates to fiber-optic couplers, and more specifically to fiber optic couplers designed to connect single mode optical fiber to legacy multimode optical fiber.
2. The Relevant Technology
The transmission of data across fiber optic cable has become increasingly common since the early 1970's. With reference to FIG. 1A, a basic fiber optic cable 100 is shown. In its simplest form, fiber optic cable 100 is a glass (silicon) strand 102 of uniform thickness having a uniform refractive index n1. This central glass strand 102, usually referred to as the core, is surrounded by a cladding 104 with refractive index n2. Light propagates through central glass strand 102 and is reflected at the interface between central glass strand 102 and cladding 104, thus guiding the light along a fiber axis 106 by total internal reflection. One property of the fiber is the numerical aperture (NA). One definition for the NA is [(n1)2+(n2)2]1/2, where n1 and n2 are defined above.
Another way to think about the NA is by defining a maximum off axis angle θ. Angle θ is the angle formed by the extreme bound meridional rays of light accepted by core 102 of fiber 100. It can be calculated using the formula θ=½ sin−1(NA). This will be discussed in more detail below.
Fiber optic cables are produced with various core diameters, depending on the application. In larger diameter fibers, currently standardized at 50 or 62.5 microns, the light can take multiple paths while traveling along the fiber. These multiple paths are known as modes, and their number is governed by the normalized frequency parameter or V-parameter. The V-parameter for a particular fiber is a function of the radius of the fiber (α), the NA, and the wavelength of the light signals being propagated through the fiber (λ) defined by the equation
  V  =      2    ⁢    π    ⁢          a      λ        ⁢          NA      .      Fibers having diameters of 50 and 62.5 microns have larger V-parameters and are thus known as multimode fibers (MMF). Because of a phenomenon known as modal dispersion, related to the differing distances traveled by light signals having the different modes, multimode fibers can carry data for only short distances, such as in and around a particular building.
In smaller diameter fibers, the V-parameter is small enough that only a single mode of light can propagate through the optical fiber. These smaller diameter fibers are known as single mode fibers (SMF). Single mode fiber is used to transmit data much longer distances, up to 100 km or longer. With regular amplification, single mode fibers can transmit optical signals as far as is necessary. Additional background information on fiber optic cables can be found in a book by B. E. A. Saleh and M. C. Teich, entitled Fundamentals of Photonics, John Wiley and Sons, New York, 1991.
FIG. 1B shows a graded index MMF 120, having a core portion 122 and a cladding portion 124. Core portion 122 has a central axis 126. Like the name implies, graded index fiber 120 has a parabolic index profile that provides a changing index of refraction, and therefore a changing NA, depending on how close to the center the incident light ray is. This is important because in the graded index case, the reported NA refers to the NA at the center. As shown in FIG. 1B, angle θ for a ray narrows as one goes from the center out to the core cladding interface. If the difference between the core and cladding refractive indices is very small, which it generally is, then the only rays accepted will be those that are nearly parallel to the core-cladding interface.
The reason for the parabolic index profile in graded index MMF is to enable the various modes to propagate down the fiber at the same speed. The higher the refractive index, the slower the light travels. The mode that travels straight down the middle of the fiber travels in the higher index portion and hence slower. The actual distance traveled is also shorter than a mode that weaves along near the core-cladding boundary. But, since the refractive index near the boundary is lower so that signals in that mode travel faster, the two modes arrive at the other end of the fiber at about the same time, cutting down on the modal dispersion.
Unfortunately, much of the optical fiber already installed in buildings throughout the country is legacy MMF (either 50 μm or 62.5 μm). For various technical reasons, connecting single mode optical fibers directly to legacy multimode optical fibers has proven difficult. In fact, it has generally been accepted that single mode optical fiber and multimode optical fiber are not compatible (see, e.g. Corning Optical Fiber White Paper #WP1160, Fiber Selection Guide for Premises Networks, David M. Bean, c. 1998, p. 4, which is incorporated herein by reference.)
Although it has been indicated that single mode optical fibers and multimode optical fibers are incompatible, there still remains a need to provide connections between legacy multimode fibers currently installed in many buildings and the single mode fibers that are the current standard.